Shocked by hull material catamaran

[Dockhead]

Originally Posted by Polux

Yes that is the way a sandwich boat is made. The sandwich, even if with thinner skins is stronger than a single skin much thicker than the two single skins but together.

Balsa as positive and negatives properties as core material. it is not a question of price. Hugely expensive brands like Contest use balsa core on their boats.

Balsa is a lot stronger than foam, although some of the newer foam materials are getting stronger.

Balsa is cheaper than foam.

Balsa is much more variable than foam.

Balsa is more prone to rot than foam.

So there are different pros and cons.

If I were building a new plastic boat, I would probably want foam in the deck -- since the ultimate strength is less important, and much greater risk of water intrusion. And balsa in the hull for the strongest possible structure.

Here's what one professional says about it:

"The reason we use cores of any type is to increase stiffness, without adding weight. The desired stiffness of a panel could also be acheived by a solid layup, and often is, but at far greater weight than sandwich construction. So what we are looking for from a core material is the best possible panel stiffnes per weight. Panel stiffness comes from the cores ability to resist compression and sheer, and balsa is superior to foam in these properties. It is denser, but to get the same stiffness from a foam core requires a thicker laminate, or a thicker core, or both, and results in a heavier panel. ATL in Queensland make Epoxy/balsa/epoxy panels, as well as epoxy/foam/epoxy panels. They have comparitive data for them on their website. www.atlcomposites.com.au"

Balsa core vs. foam core - Boat Design Forums

[Polux]

Originally Posted by Dockhead

What money Groupe Beneteau save by building cheap, they mostly pass on to the buyers. Buyers have a voracious desire to have bigger and bigger boats for less and less money -- GB are only giving people what they want.


I admire them for building so efficiently -- every boat they make, is a lot of boat for the money.


But they do seem to go too far sometimes, especially what concerns structure. The problem here is not the balsa core, but the very thin GRP layup. This is doubly tempting to cat builders because weight is such an issue, but I would not want any boat with such a thin outer skin. Yikes.

Remember the failure on the big Oyster: Look how thin is the skin:

The failure had nothing to do with the thin skin of the sandwich (that is the same in all modern Oysters of this size) but with bad calculations and bad build on the keel structure.

Sandwich hulls are like that, the skins are thin and what gives strength to the hull is the sandwich as a all. Remember that the foam saturated with resin or the balsa contribute also to the hull strength in what regards impact resistance. When delamination occur due to a big grounding with rocks hitting the hull associated with a long exposure of the core to the water the sandwish is no more and all the elements are individually weak.

Other cats, namely some expensive performance cats, that were posted in this thread, have similar sandwich with the same thin walls otherwise would not be much lighter than the Lagoon, but heavier, since they are built basically the same way.

[minaret]

I've cut into hundreds of boats with sandwich construction, and to say they all have thin skins and that's just how sandwich construction is is completely ridiculous.

[DotDun]

Originally Posted by a64pilot

Last Summer returning from the Keys we overnighted in the Venice Marina, there was a guy there that was on a larger Lagoon Cat, sorry I don't know the size. Anyway apparently he had picked up a line in one prop and was coming in on one engine, and he said when he put it in reverse, the other prop came off, so he was in the Venice inlet which is big ripp raff rocks with no engines, he drifted in to the ripp raff and it did damage his bow. He was upset cause apparently the Sheriff boat was right there watching but wouldn't help.
Anyway from just me looking at it, it was apparent that the skin had been punctured, boat wasn't going to sink or anything, but I assumed water was getting into the core, but I didn't ask as he didn't seem to be the type that listened. I say that as he was trying to convince me that water makers were stupid, what you needed was an atmospheric condenser, you could make more water for less energy that way.
Plus if he could be believed, he had enough money to buy another one anyway

Should have told him you've got one better - swap your engine for a hydrogen fuel cell as it produces drinking water as a by product.....

[Dockhead]

Originally Posted by Polux

Remember the failure on the big Oyster: Look how thin is the skin:

The failure had nothing to do with the thin skin of the sandwich (that is the same in all modern Oysters of this size) but with bad calculations and bad build on the keel structure.

Sandwich hulls are like that, the skins are thin and what gives strength to the hull is the sandwich as a all. Remember that the foam saturated with resin or the balsa contribute also to the hull strength in what regards impact resistance. When delamination occur due to a big grounding with rocks hitting the hull associated with a long exposure of the core to the water the sandwish is no more and all the elements are individually weak.

Other cats, namely some expensive performance cats, that were posted in this thread, have similar sandwich with the same thin walls otherwise would not be much lighter than the Lagoon, but heavier, since they are built basically the same way.

That Oyster is terribly built. It is built for speed, as lightly as possible, and certainly went too far, which is a shame on Oyster. It appears to have been carelessly built, as well as carelessly designed. A far cry from Oysters of yore -- the Oyster 485 I almost bought, had 80mm of solid, uncored glass in the bottom parts of the underwater sections -- built like the proverbial brick house.

Many other cored boats are built differently to that -- it depends on what the boat is built for. The outer skin of my fully cored Moody, for example, is 22mm to 25mm thick in the underwater sections -- as thick as the entire hull structure on some uncored boats -- and the last layer of cloth is Kevlar, for still more impact resistance. The designer paid a lot of attention to strength, and sacrificed cheapness and sacrificed weight, in some places, to achieve it. It's all about the design values, and intended use of the boat.

On cats, the tradeoff for weight is a lot more painful, so these tradeoffs are more difficult. Nevertheless, that Lagoon does not look well built, to my eye.

[Stumble]

It may come off as snarky, but how thick does the laminate need to be? The engineers will tell you that a thin skin is sufficient so long as it is capable of handeling the predicted loads. It's called engineering, and there is value in removing as much material as possible, subject to load requirements. I really don't get the fascination of being able to say 'my hull is 1" solid glass.' So what? It's either a waste of weight, or the loads are predicted to be insainly high.

In this case a boat was run up on a reef and survived to tell the tale. That in my book is all you can ask. And if no way she needs some core repair, that doesn't phase me at all.


As for water intrusion damaging the core.... first most boats today with cored hulls have solid glass in thru-hull areas. Second, if you need to add one where there isn't solid glass the process to protect the core isn't rocket science and is very well understood.

If your concern is puncture resistance or the ability to smash into coral without damage, then you 1) need to learn how to use a chart and 2) should be in a metal boat anyway.

[Polux]

Originally Posted by Dockhead

Balsa is a lot stronger than foam, although some of the newer foam materials are getting stronger.

Balsa is cheaper than foam.

Balsa is much more variable than foam.

Balsa is more prone to rot than foam.

So there are different pros and cons.

If I were building a new plastic boat, I would probably want foam in the deck -- since the ultimate strength is less important, and much greater risk of water intrusion. And balsa in the hull for the strongest possible structure.

Here's what one professional says about it:

"The reason we use cores of any type is to increase stiffness, without adding weight. The desired stiffness of a panel could also be acheived by a solid layup, and often is, but at far greater weight than sandwich construction. So what we are looking for from a core material is the best possible panel stiffnes per weight. Panel stiffness comes from the cores ability to resist compression and sheer, and balsa is superior to foam in these properties. It is denser, but to get the same stiffness from a foam core requires a thicker laminate, or a thicker core, or both, and results in a heavier panel. ATL in Queensland make Epoxy/balsa/epoxy panels, as well as epoxy/foam/epoxy panels. They have comparitive data for them on their website. www.atlcomposites.com.au"

Balsa core vs. foam core - Boat Design Forums

Attachment 140626

I have been watching through the years the evolution of cores on reputable boat brands. It is true that balsa have a superior stiffness and that you need a thicker foam core to compensate that, but a thicker foam core is not much more expensive to build and it provides much better properties on the case the core is infiltrated with water.



That is a trade off, I agree, but with new closed cell foam materials available almost all brands that used to use balsa are using now different types of foam. Contrary to balsa core it is not a big problem if the core is wet and the degradation of the core is incomparably smaller and the reparation much cheaper.

I had seen it done several times: They make a lot of holes on the affected area, wait sometime for the water to come out and then with a big pressure machine they blow for some days hot air on the holes. Finally when it is bone dry they inject under pressure epoxy resin till all spaces are filled. Simple and efficace without the need of any destructive work on the hull.

Here you have a good article on cores (balsa and foam).
Getting To The Core Of Composite Laminates : CompositesWorld

[noelex 77]

Originally Posted by Dockhead

That Oyster is terribly built. It is built for speed, as lightly as possible, and certainly went too far, which is a shame on Oyster. It appears to have been carelessly built, as well as carelessly designed.

Many other cored boats are built differently to that -- it depends on what the boat is built for. The outer skin of my fully cored Moody, for example, is 22mm to 25mm thick in the underwater sections -- as thick as the entire hull structure on some uncored boats -- and the last layer of cloth is Kevlar, for still more impact resistance. The designer paid a lot of attention to strength, and sacrificed cheapness and sacrificed weight, in some places, to achieve it. It's all about the design values, and intended use of the boat.

On cats, the tradeoff for weight is a lot more painful, so these tradeoffs are more difficult. Nevertheless, that Lagoon does not look well built, to my eye.

The detail is important. A cored construction with a thick outer skin such as 25 mm is, I think, a good solution. There are advantages of thermal and acoustic insulation with great overall stiffness. The drawback of delamination and water intrusion is minimised with this sort of thick skin.

Unfortunately, a cheaper and lighter construction can be made with a thicker core and much thinner skins. It seems many boatbuilders are going this way, especially when weight is critical, for example on racing monohulls or cats. Much thinner skins seem the norm.

Kevlar has advantages and drawbacks. I admire boatbuilders that use more advanced materials such as this. Unfortunately, I suspect it is often a very token amount that looks good in sales brochures but has little practical effect.

Kevlar is very puncture resistant, which is an important improvment, but is a nightmare to work with and repair, at least from an amateur's perspective. It does not absorb resin well, which raises the problems of delamination even in a solid laminate. It goes into a fuzzy ball if you try to sand it, which makes repairs more difficult.

It would be interesting to hear comments from the pros on the incorporation of a layer of kevlar into the laminate.

BTW, this is a great discussion. All too often the vital aspect of the fundemental hull construction is given little attention.

[neilpride]

Is not the same boat? like i say before, this boats are destroyed in any salvage attempt..

[Stumble]

Originally Posted by arsenelupiga

i was thinking when mentioned racing cat to have kitchen, toilet and other goodies. What you refer to is no good for even 1 week cruise.

So what you meant to say, is other than all the race boats, longer cruising cats are faster than shorter cruising cats? Sure I'll buy that.


Its a touch longer at 46', but the DNA F4 One Design racer is intended for long distance ocean racing with a top speed of ~35kn. She is also designed to foil with full crew and supplies for a 1000nm race.

[44'cruisingcat]

Originally Posted by Ilona

This showed me that there is a demand for knowing the materials boats are made of. This thread on the other hand shows me there are more people who don't care. Actually that does become clear also in the amount of these type of boats that get sold so I shouldn't be surprised

Don't care? Garbage. I PREFER to have my boat cored below the waterline. I don't want to be carting around several tonnes of useless weight which does absolutely nothing but reduce my boat's performance.

If that's not your preference fine. Plenty of people are happy to sail slowly and motor most of the time. Not me.

[a64pilot]

Are there really cored boats with 25 mm outer skins? If the outer skin is 1", then what is the core for? Insulation? I'm not trying to be sarcastic, just 1" well laid glass ought to be Hell for strong without any core.
I have drilled though my IP in a couple of places installing the generator seacock and other thru hulls, and I have not seen anything even remotely approaching that kind of thickness and I'm of course a single skin. IP's reputation is not that of a thinly built weak hull?
I would be surprised to find anywhere that the glass is 1" thick

[Polux]

Originally Posted by Dockhead

That Oyster is terribly built. It is built for speed, as lightly as possible, and certainly went too far, which is a shame on Oyster. It appears to have been carelessly built, as well as carelessly designed. A far cry from Oysters of yore -- the Oyster 485 I almost bought, had 80mm of solid, uncored glass in the bottom parts of the underwater sections -- built like the proverbial brick house.
...

I don't agree with you. I believe the Oyters are well built boats, not different from other top brands in what regards building quality and no, they are not built for speed (they are not performance boats) but to sail well that is a thing most modern boats are designed to do.

The problem on that particular boat (and on the sisterships) was an engineering error, that was assumed by Oyster. Lots of Oysters around, including many new ones, they are sailed extensively without problems.

[a64pilot]

Originally Posted by Polux

I had seen it done several times: They make a lot of holes on the affected area, wait sometime for the water to come out and then with a big pressure machine they blow for some days hot air on the holes. Finally when it is bone dry they inject under pressure epoxy resin till all spaces are filled. Simple and efficace without the need of any destructive work on the hull.

That is how dis-bonded honeycomb cores are treated with aircraft, most commonly unless the structure is very critical, then your removing skin and rebonding and or replacing the core and replacing the outer skin

[PaulinOz]

"Is not the same boat? like i say before, this boats are destroyed in any salvage attempt.."

No the boat was not destroyed, just damaged. It is my understanding that boat has since been repaired and sailing again.